Sewage treatment plants, in particular treatment plants for municipal wastewater, receive wastewater at variable flow rates and volumes that can typically be represented by a diurnal flow curve. However, under wet weather conditions, such as a severe rainstorm, wastewater intake rate and volume increase significantly due to the mixing of stormwater with wastewater. Moreover, the concentration of pollutants and the amount of dissolved oxygen of the wastewater change considerably. In particular, sewage treatment plants having groundwater infiltration and surface stormwater inflow problems experience a significant increase in sewage flow rate and volume.
If a wastewater treatment system cannot process the increased intake flow rate or volume, these very high peak wet weather inflow rates can cause solids and other pollutant breakthroughs in the sewage treatment plant due to excessively reduced hydraulic detention times in the biological treatment process reactors, and increased hydraulic and solids loading rates on the biological treatment process final clarifier.
Common practice in municipal sewage treatment plants is to switch to a “process protection” mode during wet weather inflow conditions by shutting off the mixing and/or aeration equipment in the biological treatment process reactors to allow activated sludge solids to settle out in the reactors so these activated sludge solids will not be washed into and out of the final clarifiers during peak wet weather flow rate conditions. Attempting to maintain total nitrogen and total phosphorus removal efficiency during wet weather events typically becomes very difficult for municipal sewage treatment plants with wet weather inflow problems.
It is known to use equalization basins (EQs) to store wet weather sewage influent flow surges and reduce peak throughput sewage flow rates. These EQs are commonly provided upstream of biological treatment process reactors to reduce the negative impact of variable wastewater inflow rates and pollutant loading rates; equalization basins are used to store, blend and equalize variable wastewater influent flows and loads upstream of the BNR treatment process units. Storing raw wastewater and stormwater inflow in an equalization basin (FEB) can produce odors unless the FEB is operated with aeration to transfer oxygen into the wastewater in order to maintain aerobic conditions and prevent odorous septic conditions. Aeration of the FEB, however, results in the reduction of carbon source in the raw wastewater which is needed in the downstream BNR process for nitrate-nitrogen removal by biological denitrification.
Typically, sewage treatment plants remove pollutants from the wastewater by using a series of bioreactors that are each run at different treatment conditions, i.e., aerobic treatment, anaerobic treatment, anoxic treatment, and anaerobic fermentation treatment conditions.
For example, a sewage treatment plant may treat wastewater containing biological oxygen demand (BOD) by first converting wastewater to mixed liquor while controlling dissolved oxygen (D.O.) levels. Further, the multiple biological treatments comprise deamination of proteins to form ammonia, hydrolysis of adenosine triphosphate (ATP) to form adenosine diphosphate (ADP) while releasing phosphate ions and energy and ingesting volatile fatty acids (VFAs) by polyP bacteria, and denitrification of recycled nitrates and nitrites. A detailed description of multiple biological treatments is found in U.S. Pat. No. 6,312,599 to Reid, which is herein incorporated by reference in its entirety.